A semiconductor diode conducts current in the forward direction. In the reverse direction, it blocks the current, apart from a small leakage current IL. In the forward direction, the voltage across the diode changes exponentially. For common semiconductor technologies, the forward voltage is, typically, greater than 500 mV over a large range of operating currents. As illustrated in FIG. 1, in a conventional diode, typically, the diode voltage changes with diode current as shown by the curve 101. For a resistive switch the ohmic relationship is shown by the line 103, i.e. constant resistance RON. Typical changes in the diode voltage with respect to the diode current of an active diode are shown by the curve 105. The power loss PLOSS defined as PLOSS=IDVD where ID is the diode current and VD is the diode voltage. As illustrated in FIG. 1, the diode voltage of an active diode is reduced over a large range of diode currents, thus greatly reducing the power loss of the diode. Further as illustrated in FIG. 2, the active diode (curve 105) has a much lower on resistance RON and lower diode currents than that of a conventional diode (curve 101). As a result, the active diode provides diode provides for much lower active voltages and the power loss can be minimized.
Diodes are applied in several applications, for example, battery protection, rectification, and DC-DC conversion. In all these applications, PLOSS and VD play an important role in the operation and it is desirable that the power loss PLOSS and diode voltage, VD are minimized. Often Schottky diodes are applied for this reason, because VD is, invariably, lower (e.g. 300 mV) than common semiconductor diodes. Schottky diodes however exhibit a much larger leakage current in conduction in the reverse direction, making them less useful for ultra-low power devices, in particular for high temperature applications. It is common in DC-DC converters to replace the diodes with active MOS or bipolar switches which are used with additional control circuits for forward switching and reverse switching, for example, for zero voltage detection. There have been numerous approaches to minimize the power loss of a diode by active circuits for several applications. Mostly voltage drivers MOS switches are used driven with active amplifiers, voltage drivers or comparators which require biasing currents for operating in the reverse state. This results in decreased power efficiency, especially for low power applications. In addition such voltage driven MOS switches employ “hard” voltage switching of the power switch control gate. This results in too late or too early switching from forward to reverse and reverse to forward conditions for low forward currents due to the offset, which leads to decreased power efficiency.
Further, the resulting circuits are more complex. Therefore, the use of an active diode is becoming increasingly attractive for such applications.